Ceph: get the best of your SSD with primary affinity

Using SSD drives in some part of your cluster might useful.
Specially under read oriented workloads.
Ceph has a mechanism called primary affinity, which allows you to put a higher affinity on your OSDs so they will likely be primary on some PGs.
The idea is to have reads served by the SSDs so clients can get faster reads.

Let’s see how we can configure this.
First edit your ceph.conf to enable the functionnality:

[mon]
...
mon osd allow primary affinity = true
...

Then pick up your SATA drives and set their primary affinity to 0, so they won’t become primary: